In recent years, nearly all of the secondary batteries installed in portable electronic devices such as cellular telephones and notebook personal computers are lithium secondary batteries. In addition, lithium secondary batteries are also expected to be used practically in the future as large-scale batteries of hybrid electric vehicles, electrical power load leveling systems and the like, and their importance is continuing to increase.
These lithium secondary batteries have as major constituents thereof, a positive electrode and a negative electrode, each of which contains a material capable of reversibly inserting and extracting lithium ions, and further have a separator containing a non-aqueous electrolyte or solid electrolyte.
Among these constituents, materials that have been examined for use as electrode active materials include oxides such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4) and lithium titanium oxide (Li4Ti5O12), metals such as lithium metal, lithium alloy and tin alloy, and carbon-based materials such as graphite and mesocarbon microbeads (MCMB).
Although the battery voltage is determined according to the difference in chemical potential between these materials at their lithium contents in the respective active materials, there is a possibility to obtain a large potential difference by a combination of these materials in particular, and this is one of the characteristics of lithium secondary batteries having superior energy density.
In particular, a combination of a lithium cobalt oxide (LiCoO2) active material and a carbon material which are used for electrodes enables a voltage of nearly 4 V while the combination realizes a large charge/discharge capacity, which corresponds to the amount of lithium able to be extracted from and inserted into an electrode, and the combination also results in a higher degree of safety. Therefore, the combination of these electrode materials is widely used in current lithium secondary batteries.
On the other hand, when combining electrodes containing a spinel-type lithium manganese oxide (LiMn2O4) active material and a spinel-type lithium titanium oxide (Li4Ti5O12) active material, the lithium insertion-extraction reaction tends to occur smoothly and there are fewer changes in crystal lattice volume accompanying the reaction. Therefore, it has been identified that the resulting lithium secondary battery can have a superior long-term charge/discharge cycle properties, and such a lithium secondary battery is developed for practical use.
Since chemical batteries such as lithium secondary batteries and capacitors are expected to be required to have larger size and longer life for use as electric vehicle power supplies, large-capacity backup power supplies and emergency power supplies, electrode active materials offering even higher performance (higher capacity) have come to be required by combining oxide active materials like those described above.
Among these, since titanium oxide-based active materials exhibits a voltage of about 1 to 2 V in the case of using lithium metal for the counter electrode, studies have been conducted on materials having various crystal structures regarding their potential for use as electrode active materials in negative electrode materials.
In particular, titanium dioxide active materials having a titanate bronze type crystal structure (in the present description, titanium oxides having a titanate bronze type crystal structure are abbreviated as TiO2(B)), which can achieve a smooth lithium insertion/extraction reaction comparable to that of spinel-type lithium titanium oxide while realizing a higher capacity than spinel types, are attracting attention as electrode materials (see Patent Document 1 and Non-Patent Document 1).
Among these, a process for producing TiO2(B) has been clearly determined in which H2Ti3O7 is used as a starting material, and has been clearly demonstrated to enable the synthesis of an electrode material having TiO2(B) as a main component thereof by heating in air at a temperature of 280° C. or higher.
In addition, nano-scale TiO2(B) active materials in the form of nanowires, nanotubes and the like are attracting attention as electrode materials capable of having an initial discharge capacity of more than 300 mAh/g (see Non-Patent Document 2).
However, these materials having a nano-sized particle diameter have a large irreversible capacity since a portion of lithium ions inserted by the initial insertion reaction are unable to be extracted. As a result, an initial charge/discharge efficiency (=charging capacity (amount of extracted lithium)÷ discharge capacity (amount of inserted lithium)) becomes about 73%, and this causes a problem for use as a negative electrode material in high-capacity lithium secondary batteries.
On the other hand, it has been reported that metastable phases exist during the course of a heat-treating process until the formation of TiO2(B) when using H2Ti3O7 as a starting material (see Non-Patent Document 3).
Among these, it has been reported that a phase having a chemical composition of H2Ti6O13 and a Na2Ti6O13 type framework structure exists after a heat treatment in air for one month at 140° C.; however, it has not been clearly determined what types of phases are present within a temperature range from more than 150° C. to a temperature of 280° C. at which TiO2(B) is formed.
In addition, with regard to crystal phases produced by heating H2Ti3O7 within a range of 150 to 280° C., there have also been no disclosures regarding the application of the produced crystal phases to electrode active materials.    Patent Document 1: Japanese Patent Application No. 2006-299477    Non-Patent Document 1: L. Brohan, R. Marchand, Solid State Ionics, 9-10, 419-424 (1983)    Non-Patent Document 2: A. R. Armstrong, G. Armstrong, J. Canales, R. Garcia, P. G. Bruce, Advanced Materials, 17, 862-865 (2005)    Non-Patent Document 3: T. P. Feist, P. K. Davies, J. Solid State Chem., 101, 275-295 (1992)